The present invention relates to a method for transmitting data in a control system having a plurality of spatially distributed stations which are connected to one another via a communication medium, and to a control system using the method. More particularly, the invention relates to a method and a control system for the automated control of installations or appliances, such as machining, manufacturing and/or conveyor installations.
German patent DE 199 34 514 C1 discloses a method for configuring a station (called a bus subscriber in that document) which is connected to a field bus. In particular, the document relates to the failsafe assignment of a logical address to a station which is connected to other bus subscribers/stations via a communication bus system which known as the Interbus. The Interbus is a specified field bus system used particularly in industrial automation of installations and appliances. Other known field bus systems are known as the CAN bus, the Profibus, or DeviceNet.
The known field bus systems are used to connect a plurality of spatially distributed stations/bus subscribers to one another such that they can interchange information. In this context, field bus systems are tailored particularly to the communication demands which arise from the intended use as a communication medium in control systems for the automated control of installations and appliances. Besides the simplest and most robust wiring possible, these particularly include a determinate timing response for the transmission of the data and also the ability to transport relatively small volumes of data from a large number of stations. Typically, a field bus has one or a few (intelligent) control unit(s) connected to it and also a large number of remote I/O units which pick up state signals from the installation or from the appliance via sensors and report them to the control unit(s) and/or receive control data from the superordinate control unit and operate suitable actuators on the basis thereof. By way of example, an I/O unit can pick up the signals from position switches, light barriers, rotary position transducers, inter alia, and can transmit them via the field bus to the superordinate control unit. The control unit, frequently a programmable logic controller (PLC), takes these process variables as a basis for determining control data for actuators, such as solenoid valves, contactors, drives, inter alia. The I/O units receive the control data from the superordinate controller via the field bus and operate the actuators.
The Interbus cited at the outset operates in the manner of a large shift register whose individual storage locations are in the connected stations. What is known as a bus master, which is frequently physically arranged in the superordinate control unit, generates a data frame having a number of data fields which corresponds to the number of storage locations in the “shift register”. This data frame is sent from the bus master to the stations connected in series and in so doing is forwarded data field by data field from one station to the next. The last station in the series returns the data frame to the bus master, so that a ring structure is ultimately obtained. When a start word generated by the bus master arrives on the bus master again after passing through the ring structure and no transmission errors are identified in a subsequent checksum evaluation, the bus master uses a control signal to signal all the connected stations that they need to accept the data which are then in their respective shift registers for further processing. In addition, the individual stations “empty” their internal shift registers filled with transmission data by transmitting the data stored therein to the next station in the ring when the bus master initiates a new data circulation. One characteristic of the Interbus is therefore that the number of data fields in the circulating data frame is equal to the number of storage locations in the connected stations. In addition, communication actually takes place only between each individual station and the bus master. Interconnecting traffic between two stations which do not have bus master functionality is possible only from one transmission cycle to the next by virtue of the sending station first of all transmitting its data to the bus master and the bus master forwarding the data to the receiving station in a second data cycle. An advantage of the Interbus concept is the deterministic timing response, i.e. the time required for transmitting a piece of information can be predicted with a high degree of certainty. In addition, no collisions can occur between competing messages.
In contrast to this, collisions are basically possible in the case of what are known as message-oriented field bus systems, such as the CAN bus, since the individual stations can generate and send data messages independently. In the case of the CAN bus, such collisions are resolved by virtue of the stations having different priorities, where a station having a higher priority asserts its authority in the event of a collision. For the station having a lower priority, however, this means that it is at least temporarily prevented from sending a message. To implement a deterministic timing response, it is therefore necessary to limit the maximum utilization level of the field bus, since the probability of collisions rises as the utilization level of the bus increases. On the other hand, CAN bus-based control systems provide a higher level of flexibility, since interconnecting traffic is possible, in principle, without the interposition of a bus master. This particularly means that safety-related data, such as an emergency off command, can be transmitted more quickly, even as a broadcast telegram if appropriate.
Outside of the specific field bus technology, communication networks based on what is known as the Ethernet standard have become widely used as a result of the Internet and the networking of personal computers. In Ethernet networks, each subscriber (each station) has an individual address, what is known as the MAC address. In principle, any station can send a message at any time. Each sending station monitors the connecting line to determine whether the message sent can also be read in uncorrupted form, which would not be the case if there were a collision with a simultaneously sending station. In the event of a collision, each station sends its transmission data again after a randomly selected time period has elapsed. Due to the widespread use, Ethernet networks have the advantage that the relevant hardware components are very inexpensive. However, they do not have a deterministic timing response and, moreover, are more optimized for acyclic transmission of relatively large volumes of data by few connected stations. By contrast, control systems primarily require cyclic data transmission. However, the inexpensive hardware components mean that for some years there have been efforts to use Ethernet technologies for the communication between the stations in a control system for controlling a technical installation or a technical appliance too. Principles and providers of corresponding components are described by way of example in the German publication “Industrial Ethernet”, which is available under ISBN 3-8259-1925-0 from Vogel-Industrie Medien GmbH und Co. KG in 97064 Würzburg.
The previously proposed approaches to using Ethernet components in automated control systems are not optimum, however. Difficulties arise particularly for the transmission of safety-related control data, such as the transmission of an emergency off signal or the transmission of a shut down command which is subsequently generated by the control unit for a drive. Data transmission with short cycle times, which is needed for drive control, for example, is also difficult to implement on the basis of Ethernet components.